Method validation and clinical utility of chromogenic factor VIII assay compared to one-stage assay

Guidance for establishing a factor VIII testing protocol for the myriad of factor VIII products

INTRODUCTION

Management of hemophilia A has changed significantly in the past few years with the expansion of new and/or modified products as treatment options. Unfortunately, many of the standard factor VIII assays do not always accurately measure all available treatment products; therefore, the laboratory must investigate various assay algorithms to ensure the reporting of the correct results.

METHODS

Requirements for factor testing, diagnosis and severity levels, product testing, factor VIII inhibitor detection and titers, are evaluated, and potential algorithms are created for optimal assessment of patients with hemophilia A.

RESULTS

The potential for inaccurate result reporting for patients with hemophilia A or those being treated with the myriad of products has left many laboratories uncertain as to which assay algorithm to implement to ensure reporting the correct results for all products used in their hemophilia program. Algorithms for using either One-stage Clotting assays or Chromogenic assays or a combination of both types of assays are presented for each laboratory to implement based on their clinical situation.

CONCLUSIONS

Several algorithms are considered based on the needs of the clinical providers and their patients. Each laboratory must select a testing algorithm that is cost-effective and within available resources, yet that encompasses the needs of their providers and patients. Laboratory personnel must consider all assay uses (factor VIII levels, different products, interfering products, and inhibitor titers) in determining the best algorithm for their laboratory. This paper is a starting guide for developing the best factor VIII testing assays and protocols for your laboratory.

A critical appraisal of one-stage and chromogenic assays of factor VIII activity

Accurate and precise potency determination by manufacturers of different types of factor VIII product (plasma-derived and recombinant FVIII [rFVIII]) is vital to clinicians and patients using FVIII concentrates. A separate, but related, requirement is ascertaining the FVIII activity levels in clinical samples for diagnosing and treating hemophilia A. The one-stage clotting assay (OSA) and the chromogenic substrate assay (CSA) are the main assays used for these measurements, with both assays being used for potency assignments, and the OSA also being widely used for clinical monitoring. Although the assays can produce concordant results, discrepancies often occur, e.g. when measuring FVIII levels in patients with mild or moderate hemophilia A, or when assaying high-purity FVIII products. Modifications to rFVIII proteins, such as B-domain deletion (BDD), and technologies for improving the pharmacokinetic profile of rFVIII may exacerbate assay discrepancies. The CSA appears to be essentially unaffected by these modifications. However, the OSA underestimates the FVIII activity levels and therapeutic potential of some further modified BDD rFVIII products, especially those conjugated to poly(ethylene glycol); the extent of the effects is dependent on the specific OSA reagents used. Although the OSA remains the preferred choice for clinical monitoring in Europe and the USA, an awareness of the limitations of that assay has prompted more laboratories to adopt the CSA.